I. Field of the Invention
The apparatus of this invention is an antenna system useful in satellite-to-ground communications in the microwave frequency spectrum, especially the band around 4 gHz. Antenna systems of the general type under consideration have been in use for many years and consist principally of a waveguide feed array facing toward a parabolic dish reflector and located at the focus thereof but offset from the principal axis of the reflector. In the transmit mode the waveguide feed array propagates microwave energy through the short region of space to the parabolic reflector from which it is reflected into a propagating beam of desired shape directed toward a selected region on Earth. Similarly, signals beamed toward a satellite and received by the reflector dish in the proper orientation thereto can be directed toward the waveguide array for reception. The satellite with which such antenna systems are used can be provided with solar collectors and batteries for continuously supplying power, and with the requisite electronics to turn the antenna system into a complete reception and transmission station for repeater or other purposes. By suitably shaping the beam pattern of the antenna system, widely separated zones on the face of the Earth can be placed in excellent mutual communication with one another through the medium of the satellite which acts as a repeater.
Because the use of such communication satellites has become so accepted for a wide variety of purposes ranging from the transmission of entertainment programs to uses in ground surveillance and meteorological data gathering, it has become desirable in recent years to expand the number of communication channels which can be handled by a single antenna system. In the past, a 10% bandwidth has been considered adequate for such antenna systems, although wider bandwidth would, of course, permit the accommodation of more communications channels per satellite.
II. Description of the Prior Art
U.S. Pat. No. 4,115,782 issued Sept. 19, 1978, having common assignment with the present application and including a common coinventor therewith, describes a microwave antenna system of the general type outlined above which successfully propagated over a 10% bandwidth and maintained excellent rejection of interfering cross-polarization. Although this performance was considered excellent at the time of this earlier patent, recent developments and increased demands have required that the bandwidth be further increased to the maximum permissible, i.e., 20%.
Studies made to determine the feasibility of extending the bandwidth of the prior art device of the above patent to 20% produced the result that such bandwidth extension could be obtained only at the expense of a considerable degradation of performance, especially with respect to maintaining low cross-polarization. Further studies indicate that a considerable alteration of the feed array would be necessary in order to secure the desired bandwidth while maintaining low cross-polarization.
The antenna systems of U.S. Pat. No. 4,115,782 utilized a closely packed feed array consisting of square wave guides. While this feed array performed admirably over a 10% bandwidth and provided excellent gain because of the close packing possible when using a square or rectangular cross-section for the individual waveguides, the researches which led to the present invention disclosed that such a waveguide array was incapable by any known means of providing both the bandwidth and cross-polarization performance desired.
U.S. Pat. No. 4,090,203 to James W. Duncan issued May 16, 1978, and covers various antenna systems of the general type under consideration wherein an offset feed array is employed with a generally paraboloidal reflector. The Duncan patent employs circular waveguides as feed elements arrayed in a variety of configurations as disclosed in his FIGS. 11-15, but does not deal with the critical problems involved in extending bandwidth beyond that which was routinely achieved in the prior art systems existant at the time of the Duncan device. Duncan's patent is principally concerned with techniques to enhance the suppression of side lobes to thereby secure a more nearly Gaussian distribution of energy in the beam.
U.S. Pat. No. 3,790,941 to Malcom Chivers et al, deals with an antenna tracking system employing a single circular waveguide together with a polarizer and a rectangular-to-circular transition. However, Chivers et al does not deal with the problems associated with enhancing bandwidth while maintaining low cross-polarization in a multiple feed array such as found in the present invention. Consequently, there are no teachings to be found within Chivers et al as to the proper arrangement of such a feed array, or as to how to overcome problems of cross-polarization and mutual coupling when such an array is provided.
U.S. Pat. No. 3,936,835 to Harry Richard Falin concerns feed systems for multiple beam antennas in which arrays of circular waveguides of different configurations are shown. However, no means are illustrated or described in this patent for securing the broad bandwidth and low cross-polarization achieved by the means of the present invention.
U.S. Pat. No. 4,122,446 to Lawrence H. Hanson illustrates a type of transition in circular wave guide consisting of a step segment 12 for achieving transition with minimum reflection loss and VSWR. However, the patent contains nothing else instructive in the arts of the present invention.
U.S. Pat. No. 3,864,683 to Gunter Morz is directed to an automatic direction finding system for orienting a microwave antenna and is unrelated to the concerns of the present invention except for its inclusion of a coax-to-waveguide coupler and a square-to-circular transition for use in waveguide systems.
U.S. Pat. No. 3,680,138 to Harold A. Wheeler illustrates a means of mode control in a multiple array of circular waveguides including a cross-mode reflector 7 which is claimed to provide linear polarization even though the apertures of the waveguides are circular.
The following U.S. Pat. Nos. are cited as of general background interest in an evaluation of the present invention:
U.S. 3,940,772 issued Feb. 24, 1976
U.S. 3,811,129 issued May 14, 1974
U.S. 3,271,776 issued Sept. 6, 1966
U.S. 3,553,706 issued Jan. 5, 1971
U.S. 3,564,552 issued Feb. 16, 1971
U.S. 3,706,998 issued Dec. 19, 1972